BIG PHYSICS, BIG QUESTIONS –

Moon’s hidden craters detected by gravity mapper

There are obvious craters like Flamsteed P (bottom) and then there are those disguised by lava flows (larger ring)

NASA

By Matt Reynolds

The moon has buried scars. Maps of its gravity have confirmed the existence of hidden, ancient craters, long since filled in by lava flows and rising lunar mantle.

By combining gravity-mapping data with their own mathematical models, Jay Melosh at Purdue University in West Lafayette, Indiana, and his colleagues have confirmed the existence of two underground craters, one completely buried beneath the Sea of Tranquility. The approach could let us map every single impact that punctured the moon’s surface since its crust formed around 4.2 billion years ago, Melosh says.

Astronomers have known about these buried craters since the early days of lunar science, says Jeffrey Andrews-Hanna at the University of Arizona in Tucson, who was not involved in the new work. “We can still see their rims poking up above the maria like islands in a sea of frozen lava,” he says.

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Last year, Alex Evans at the University of Arizona and colleagues used data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission to find evidence of over 100 craters buried beneath seas of basalt formed by ancient volcanic eruptions.

GRAIL consisted of twin spacecraft, called Ebb and Flow, that orbited the moon for nine months in 2012. Measuring small changes in their acceleration allowed scientists to map the moon’s gravity. That in turn gives insights into variations in the density of the lunar surface and interior.

“Gravity is sensitive to the distribution of mass beneath the surface, and so we can use it to see into the inside of the moon,” says Andrews-Hanna. Measurements from GRAIL also revealed buried rift valleys, structures underneath ancient volcanoes and other formations caused by volcanic activity.

Earhart and Ashoka

Melosh and his colleagues built on this work. They were searching the GRAIL data for traces of underground lava tubes when they came across the two large buried craters that had only been hinted at in Evans’s research.

One crater, which the team call Earhart, measures around 200 kilometres in diameter. Located in the north-eastern part of the moon’s near side, it is almost completely masked by a later impact and subsequent lava flooding. It was probably created by an asteroid impact around three billion years ago, after the moon formed a crust but before it significantly cooled.

Melosh estimates that the asteroid made a crater 40 or 50 kilometres deep, which was then filled in by a combination of lava flow from volcanoes and the moon’s mantle pushing up the thin crust. The Earhart impact left behind a small, visible trace of a crater rim which hinted at the existence of crater beneath.

The team also discovered a slightly smaller buried crater 160 kilometres in diameter, which they called the Ashoka Anomaly.

Further analysis of these buried craters could reveal more about the lunar surface beneath the vast plains of volcanic deposits, Andrews-Hanna says. “Like peeling back the layers of an onion, we can use gravity data to see below the surface and learn things like the age of the layers beneath,” he says.